Content of total ascorbic acid (dehydroascorbic acid and L-ascorbic acid) in fresh and preserved sweet potatoes by boiled cooking were compared with the raw sweet potato. By preservation, content of total ascorbic acid in sweet potato were degreased to 75.9%. On heating by boiling water, content of total ascorbic acid in fresh sweet potato were almost the same (108%) as those in the raw potato. On heating by boiling water, contents of total ascorbic acid in preserved sweet potato were decreased to 67.2%. Accordingly, L-ascorbic acid was decomposed to dehydroascorbic acid and the others by presevation(, and dehydroascorbic acid was decomposed by boiling temperature).